This invention pertains to alkoxy-type derivatives of Group 3b metals including trivalent rare earth metals, exemplified by cerium, such as alkoxides, silyloxides, or alkanolatoamine compounds, and methods for preparing these compounds. 2. Discussion of Background
Polyvalent metal alkoxy derivatives are versatile organometallic compounds. The alkoxides, for instance, have been used as paint additives, water repellents, adhesion promoters, mordants, sizing agents in enamel compositions, catalysts, precursors for ceramics and fibers and also as intermediates in the synthesis of other metal-organic compounds. There are four general preparative methods for metal alkoxides all under anhydrous conditions, as follows:
A. By reaction of the corresponding alcohol and metal, such as the alkali metals, alkaline earth metals, and aluminum, with the assistance of an alkaline earth or acidic catalyst.
B. By reaction of the corresponding alcohol with the oxides and hydroxides of the metal, for instance NaOH or Na.sub.2 O, V.sub.2 O.sub.5 and MoO.sub.3.2H.sub.2 O.
C. By reaction of the corresponding alcohol and metal halide in the presence of an anhydrous base. A typical example is the preparation of Th(OR).sub.4 or Zr(OR).sub.4 : EQU ThCl.sub.4.4ROH+4NaOR.fwdarw.Th(OR).sub.4 +4NaCl EQU ZrCl.sub.4 +4ROH+4NH.sub.3 .fwdarw.Zr(OR).sub.4 +NH.sub.4 Cl
The reaction can be used for preparing alkoxides of titanium, hafnium, germanium, niobium, tantalum, aluminum and tin.
D. By transetherification of the metal alkoxides of lower alcohols, such as the methoxides, ethoxides or isopropoxides, with a higher alcohol.
Method A is exemplified for a number of yttrium, lanthanum and other lanthanide alkoxides by L. Brown and K. Mazdiyasni in Organic Chemistry, (1970) p. 2783. The reaction, previously thought to be useful only for the alkali metals magnesium and aluminum, was extended by them to the synthesis of yttrium and all of the lanthanide isopropoxides. For the lower lanthanides, such as lanthanum, cerium, praesodymium and neodymium, a mixture of HgCl.sub.2, and Hg(C.sub.2 H.sub.3 O.sub.2).sub.2 or HgCl.sub.2 is used as a catalyst, to increase both the rate of reaction and percent yield Generally, 5 g of the metal turnings is reacted with about 300 ml of isopropyl alcohol at reflux temperature for about 24 hours and in the presence of a small amount of Hg salt catalyst. The yields are said to be 75% or better.
Most of the other examples in the literature of the preparation of alkoxides of lanthanides refer to the use of corresponding metal halides. In some cases, a complex LaCl.sub.3. 3ROH is preferred to the LaCl.sub.3 (Misra et al., Austr. J. Chem. 21 p. 797 (1978) and Mehrotra and Batwara, Inorganic Chem. 9 p. 2505 (1970).
An interesting variation of Method D is mentioned by Tripathi, Batwara and Mehrotra, J.C.S.A. (1976) p. 991. Lower ytterbium alkoxides (such as methoxide and ethoxide) were synthesized from ytterbium isopropoxide, by transetherification with methanol or ethanol. Owing to their sparing solubility, these alcohols were removed by precipitation as the reaction proceeded, driving the transetherification to completion.
In general, Methods A, B and C are only suited for preparation of the lower alkoxides, such as the methoxides, ethoxides and isopropoxides, since the reactivity of higher alcohols diminishes with increase in their molecular weights. The higher alkoxides are better prepared by Method D, which is a two-step process.
U.S. Pat. Nos. 4,489,000 and 4,492,655 to Gradeff and Schreiber describe the preparation of tetravalent cerium alkoxides from ceric ammonium nitrate and are incorporated by reference. U.S. Ser. No. 895,560 relates to tetravalent alkanolatoamine derivatives and method for preparation from ceric ammonium nitrate and is incorporated by reference. U.S. Pat. No. 4,663,439 to Gradeff and Schreiber disclose a process for preparing ceric alkoxides and is incorporated by reference.
Modern separation techniques for lanthanides yield these elements primarily as chlorides or nitrates in aqueous solution. They are used to make all other derivatives such as carbonates, hydroxides, oxides, etc. They can also be used to make some of the organic derivatives, such as high alkyl carboxylates or acetyl acetonates. Anhydrous lanthanide inorganic salts, however, are essential for making alkoxides, Ln-carbon bond derivatives and a host of others, as well as for the production of metals by electrolytic and metallothermic processes. Oxides can be made anhydrous, but their low solubility and limited reactivity precludes their use in many areas. The halides are the only class of compounds that are being used as a source of anhydrous species suitable for these purposes. The easiest ones to make are the fluorides, but they have only limited use in syntheses due to extremely low solubility and reactivity. Among the most difficult ones to dehydrate are the iodides. The bromides and chlorides present a similar degree of difficulty which is greater than the fluorides but less than the iodides. In view of economic and environmental considerations, the chlorides are the most sought anhydrous lanthanide salts. This includes cerium, the most abundant among the lanthanides.
The halides which separate from their aqueous solutions, usually retain 6-7 moles of water. After the unbound water has been removed, most of the bound water can be removed by careful dehydration below 100.degree. C. It is extremely difficult, however, to remove the last mole of water without decomposing the halide.
Dehydration of lanthanon chloride hydrates done with HCl gas is a tedious process. Dehydration using ammonium chloride in addition to HCl, and at temperatures below about 200.degree.-300.degree. has shown better results. Oxides have been reacted with sulfur monochloride and chlorine or sulfur monochloride alone. Thionyl chloride, carbon tetrachloride and phosgene have also been used as reagents. Preparation of the most commonly used anhydrous salt of lanthanides, the chloride, is not an easy task. (Chem. Rev., 1962, pp. 503-511).
U.S. Pat. No. 4,492,655 to Gradeff and Schreiber discloses the preparation of Cerium(III) cyclopentadienyl derivatives using Ceric Ammonium Nitrate and Na-cyclopentadienyl. Use of a nitrate represented a departure from conventional routes. Ceric ammonium nitrate was advantageous in that it was easily obtained in anhydrous form. The process of that invention comprises slowly adding alkali metal cyclopentadienide to a solution of ceric ammonium nitrate, and forming in sequence mono- to tricyclopentadienyl cerium. The overall reaction equation is shown as EQU 2[Ce(NO.sub.3).sub.4.2NH.sub.4 NO.sub.3 ]+12NaCp.fwdarw.2Ce(Cp).sub.3 +4CpH+(Cp).sub.2 +12NaNO.sub.3 +4NH.sub.3.
The "in situ" reduction step is shown to proceed via two possible pathways. EQU Ce(NO.sub.3).sub.4 +4NaCp.fwdarw.[Ce(Cp).sub.4 ]+4NaNO.sub.3 [2Ce(Cp).sub.4 ].fwdarw.[2Ce(Cp).sub.3 ]+Cp.sub.2 (a) EQU Ce(NO.sub.3).sub.6.2NH.sub.4 +NaCp.fwdarw.[Ce.sup.+4 Cp(NO.sub.3).sub.5.2NH.sub.4 ]+NaNO.sub.3 .fwdarw.Ce.sup.+3 (NO.sub.3).sub.5.2NH.sub.4 +1/2Cp.Cp (b)
The above "in situ" reaction has utility in cases where reagent is a good reducing agent whereby part of the reagent is consumed in the reduction of Ce.sup.+4 to Ce.sup.+3. In cases as the above referred one, this can be rather expensive.